Matt,First you need to know how to convert psi (Pound per Square inch) to Bars. 1 Bar = 14.5 psi. So if you are reading 48 psi at the low side, it is 48/14.5 = 3.3 Bars. 250 psi at the high side is 250/14.5 = 17.2 Bars.
This is what you tell me what you read with your gauges.
Now find the graphs for the 210.055 chassis (this is 1997 E320 which has the same a/c system as your car) which are the right column of graphs. Then if the ambient temperature is 25 degrees C, find 25 degrees C in the horizontal line (x or abscissa plane), and draw a vertical line over all 4 graphs, and note the values in the vertical lines.
4-6 degrees C for the air temp from the center vent
1.8 Bars for the low side pressure
10-14 Bars of High side pressure
94 degrees C for the refrigerant temperature.
So if you are reading 3.3 Bars at the low side, it is 3.3 - 1.8 = 1.5 Bars higher than what it should be. 1.8 Bars is around 26 psi, and you are reading 48 psi.
So you have a high low pressure side problem. If you look at the interpretations, it could be a bad compressor, or a continuously open expansion valve. For the latter, you get ice cold air coming from the vents first, then it will get warmer and warmer, as the low side pressure starts building up. Sometimes a defective expansion valve will cause the compressor valve to go bad, as when you have more refrigerant in the liquid form at the low side, and the compressors do not like to compress refrigerants in liquid form.
Regarding the line blockage, over time, you get crud in the system, especially if you introduce moisture, air when you charge the system up (without purging the charge hoses of moist air). With refrigerant, oil and water and other contaminants, you will have a plugged up receiver / drier and partially clogged condenser. This will increase the high side pressure, as the flow is restricted. This is one of the problems of relying on the pressure gauge of the climate control system which reads the pressure after the drier, not before. So if the drier is plugged up and restricting flow, the gauge will always read less pressure than the actual gauge pressure at the high side valve. This is why it is important to use gauges, instead of completely relying on climate control system readings.
I get it now Ambient at 25 C approx 80% RH)
Center air outlet temp is much higher than the 5-7 C expected(41 F)
Clearly I have 1 BAR higher pressure on the LO side.
HI pressure is also higher than expected at around 210.
Not sure what the refrigerant temp is -- I guess I can read from the diagnostics.
Based on this I definitely need a new compressor and as you have suggested, and I may as well replace some of the other components as you have also suggested.
Now I need to assess if I can do these by myself, through some DIY.